critical stress for structural members

huangyhg

critical stress for structural members
i am trying to learn how to calculate basic structural members with the help of "limit states design in structural steel" textbook.  
for lateraly unbraced beams, mr is smaller than or equal to phi x s x fcr,
where  mr is moment resistance
       phi is resistance factor
       s is elastic section modulus
       fcr is critical stress
how do i find fcr???  equations or tables.  the textbook does not say how nor gives any examples.
thank you in advance.
mr = phi x s x fy
where fy = specified minimum yield stress (which is usually 300mpa or 350 mpa depending on the type of steel being used)
thank you civeng!
meckman
i presume you are starting to learn lrfd.
as you will see after, depending on the type of sections and their availability to develope plastic hinges, the elastic modulus, will turn into the plastic modulus. this is the real meaning of a limit state behaviour.
for the plastic modulus you will find them easily in section data tables.
please enjoy the limit state design, because that is the way.
regards,
cv
cv
it looks like you are studuying in the uk ?
if so, take a look at the bs5950 in the first pages, you have steel grades, and corresponding stresses for each steel grade depending on the thickness of the steel member.
usually py = 275, and you should use something around 255 to 245 n/mm2
hope it helps...